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Ngày đăng: 30/11/2016, 16:19

Sedimentary and Metamorphic Rock • Sediments are pieces of solid material that have been deposited on Earth’s surface by wind, water, ice, gravity, or chemical precipitation • When sediments become cemented together, they form sedimentary rocks • Chemical weathering occurs when the minerals in a rock are dissolved • Physical weathering- minerals become chemically unchanged, rock fragments break off • Weathering produces rock and mineral fragments known as clastic sediments • Deposition -sediments are laid down on the ground or sink to the bottom of bodies of water – Water and wind deposits sorted into layers – Glaciers, landslides – unsorted deposits • Lithification is the physical and chemical processes that transform sediments into sedimentary rocks • Cementation occurs when mineral growth cements sediment grains together into solid rock Types of Sedimentary Rocks • Clastic sedimentary rocks – deposits of loose sediments sifted according to the sizes of their particles – Conglomerates, breccias – Sandstone (medium grained clastic) • Chemical sedimentary rocks form when crystal grains precipitate out of solution and settle to the bottom of a body of water – evaporates – calcite, halite, gypsum • Biochemical sedimentary rocks are formed from the remains of once-living things – During burial and lithification, calcium carbonate precipitates out of the water, crystallizes between the grains of carbonate sediment, and forms limestone – Coal is composed almost entirely of carbon and can be burned for fuel • Fossils provide a geologic “snapshot” of surface conditions in earth’s past • For example, location and direction of flow of ancient rivers, the wave or wind direction over lakes and deserts, and ancient shoreline positions all can be indicated Metamorphic Rocks • Cause of metamorphism-When high temperature and pressure combine to alter the texture, mineralogy, or chemical composition of a rock without melting it, a metamorphic rock forms • High temperature and pressure combine to alter the texture, mineralogy, or chemical composition of a rock without melting it, causing a metamorphic rock to form • When high temperature and pressure affect large regions of earth’s crust, they produce large belts of regional metamorphism • Belts are divided into zones based upon the mineral groups found in the rocks • Contact metamorphism-When molten rocks,such as those in an igneous intrusion, come in contact with solid rock – high temperature and moderate-to-low pressure – Minerals that crystallize at high temperatures are found closest to the intrusion • Hydrothermal metamorphism-When very hot water reacts with rock and alters its chemistry • Foliated metamorphic rocks –Wavy layers and bands of minerals – flat, needlelike crystals, long axes perpendicular to the pressure • Nonfoliated – no mineral grains with long axes, blocky crystal shapes • Porphyroblasts – large crystals of new metamorphic minerals The Rock Cycle • Igneous rocks crystallize from magma • A metamorphic rock may be changed into another metamorphic rock or melted to form an igneous rock • Sandstone might become uplifted and weathered back into sediments [...]...The Rock Cycle • Igneous rocks crystallize from magma • A metamorphic rock may be changed into another metamorphic rock or melted to form an igneous rock • Sandstone might become uplifted and weathered back into sediments